You will need to have macports installed and working.
Basically, you are going to disable postfix, install the SSMTP port, configure SSMTP, and then (here’s the part that is not documented) replace the sendmail executable with a link to ssmtp.
Here are the commands.
$ sudo /bin/launchctl unload -w /System/Library/LaunchDaemons/org.postfix.master.plist
$ sudo port install ssmtp
$ sudo cp /opt/local/etc/ssmtp/ssmtp.conf.sample /opt/local/etc/ssmtp/ssmtp.conf
$ sudo edit /opt/local/etc/ssmtp/ssmtp.conf
$ sudo mv /usr/sbin/sendmail /usr/sbin/sendmail.orig
$ sudo ln -s /opt/local/sbin/ssmtp /usr/sbin/sendmail
You should now be able to…
$ mail firstname.lastname@example.org
Now tail the mail log to verify that your mail was sent: